Most submarine cables have only two terminal points, i.e. one at each end. However, the high capacity of optical systems makes it possible for a submarine cable, e.g. a transatlantic or transpacific cable, to serve a very large land area and it may be convenient to provide a plurality of terminal points connected by submerged optical cables to submerged optical junctions (which have a plurality of input fibres and one output fibre).
In order to keep submerged equipment as simple as possible, it has been proposed that the junctions are all passive. That is the output is simply a concatenation of the various inputs with phase and amplitude relationships preserved. It is, therefore, important that input pulses should not co-incide, i.e. it is important that phase relationships at the junction be correct.
The transmitters may be widely separated geographically but, even in these circumstances, a fixed phase relationship could be established. However, the relationship at the junction involves the transmission times along the optical fibres to the junction. The fibres are subjected to the ambient temperatures of the sea and therefore the temperature varies, e.g. diurnal and seasonal variations. The refractive index, and hence the speed of transmission, is dependent on temperature. Thus phase relationships at a junction cannot be controlled only from the transmitters.